In the present study, the biosynthesis of ZnONPs utilizing Portulaca oleracea seeds aqueous extract has been reported. The reported method is a green, plant-extract mediated and low-cost approach that is capable of synthesizing ZnONPs at pH 7 with heating at 60 oC for one hour. To demonstrate the efficiency of the capping bioorganic molecules on ZnO nanoparticles, a part of the product was dried at 60 oC (denoted ZnO-60) and the other part was annealed at 500 oC (denoted ZnO-500). Both synthesized ZnONPs was crystalline in nature and hexagonal in shape with an average size of 14 nm and 34.5 nm for ZnO-60 and ZnO-500, respectively, as evidenced by XRD and TEM analysis. The synthesized nanoparticles showed high antioxidant, good antibacterial activity, which has inhibited the growth of Helicobacter pylori (ATCC 700392) and Methicillin-Resistant Staphylococcus aureus bacteria and anticancer activity for HCT-116 cells with a higher efficiency of ZnO-60 than ZnO-500.